tvl-depot/absl/debugging/internal/address_is_readable.cc
Abseil Team 0d5ce2797e Export of internal Abseil changes
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20405fc394419d434d3ea09b2e62e4edcb282421 by Christian Blichmann <cblichmann@google.com>:

Include `status` subdirectory in main `CMakeLists.txt`

PiperOrigin-RevId: 297125966

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101087af9689612bdda679ee0869e5cde4472244 by Matt Kulukundis <kfm@google.com>:

Fix typo

PiperOrigin-RevId: 296991360

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55ff5bc6970d46214c0459d3a7a23973c7dc69b9 by Andy Getzendanner <durandal@google.com>:

Extract logging's ErrnoSaver to absl::base_internal and use it in a couple other places.

PiperOrigin-RevId: 296969168

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b7cd7550297d53766576f751436617200c65831b by Abseil Team <absl-team@google.com>:

Internal change

PiperOrigin-RevId: 296951474

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8c04c73fc53f9a09c3e2400812b931157f35fe07 by Andy Soffer <asoffer@google.com>:

Auto-generate list of files in the DLL, and add new build targets to the DLL.

PiperOrigin-RevId: 296932061

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2f77829e196094f1addefd8ac2ac9e398c5b6100 by Andy Soffer <asoffer@google.com>:

Fix bug introduced by DLL where we couldn't build shared libraries on
non-windows platforms.

Fixes #623

PiperOrigin-RevId: 296919347

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b768163dbbff0c561b9dff0218a699e5e4fd33f3 by Abseil Team <absl-team@google.com>:

typo: microprosessor

PiperOrigin-RevId: 296904933

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b185da0dac44c91855373f0723df9242cbfb3db3 by Matthew Brown <matthewbr@google.com>:

Internal cleanup

PiperOrigin-RevId: 296509711
GitOrigin-RevId: 20405fc394419d434d3ea09b2e62e4edcb282421
Change-Id: I55c8eba6f353ceb337455ae144ab743ea21edbef
2020-02-25 13:12:47 -05:00

138 lines
5.2 KiB
C++

// Copyright 2017 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// base::AddressIsReadable() probes an address to see whether it is readable,
// without faulting.
#include "absl/debugging/internal/address_is_readable.h"
#if !defined(__linux__) || defined(__ANDROID__)
namespace absl {
ABSL_NAMESPACE_BEGIN
namespace debugging_internal {
// On platforms other than Linux, just return true.
bool AddressIsReadable(const void* /* addr */) { return true; }
} // namespace debugging_internal
ABSL_NAMESPACE_END
} // namespace absl
#else
#include <fcntl.h>
#include <sys/syscall.h>
#include <unistd.h>
#include <atomic>
#include <cerrno>
#include <cstdint>
#include "absl/base/internal/errno_saver.h"
#include "absl/base/internal/raw_logging.h"
namespace absl {
ABSL_NAMESPACE_BEGIN
namespace debugging_internal {
// Pack a pid and two file descriptors into a 64-bit word,
// using 16, 24, and 24 bits for each respectively.
static uint64_t Pack(uint64_t pid, uint64_t read_fd, uint64_t write_fd) {
ABSL_RAW_CHECK((read_fd >> 24) == 0 && (write_fd >> 24) == 0,
"fd out of range");
return (pid << 48) | ((read_fd & 0xffffff) << 24) | (write_fd & 0xffffff);
}
// Unpack x into a pid and two file descriptors, where x was created with
// Pack().
static void Unpack(uint64_t x, int *pid, int *read_fd, int *write_fd) {
*pid = x >> 48;
*read_fd = (x >> 24) & 0xffffff;
*write_fd = x & 0xffffff;
}
// Return whether the byte at *addr is readable, without faulting.
// Save and restores errno. Returns true on systems where
// unimplemented.
// This is a namespace-scoped variable for correct zero-initialization.
static std::atomic<uint64_t> pid_and_fds; // initially 0, an invalid pid.
bool AddressIsReadable(const void *addr) {
absl::base_internal::ErrnoSaver errno_saver;
// We test whether a byte is readable by using write(). Normally, this would
// be done via a cached file descriptor to /dev/null, but linux fails to
// check whether the byte is readable when the destination is /dev/null, so
// we use a cached pipe. We store the pid of the process that created the
// pipe to handle the case where a process forks, and the child closes all
// the file descriptors and then calls this routine. This is not perfect:
// the child could use the routine, then close all file descriptors and then
// use this routine again. But the likely use of this routine is when
// crashing, to test the validity of pages when dumping the stack. Beware
// that we may leak file descriptors, but we're unlikely to leak many.
int bytes_written;
int current_pid = getpid() & 0xffff; // we use only the low order 16 bits
do { // until we do not get EBADF trying to use file descriptors
int pid;
int read_fd;
int write_fd;
uint64_t local_pid_and_fds = pid_and_fds.load(std::memory_order_relaxed);
Unpack(local_pid_and_fds, &pid, &read_fd, &write_fd);
while (current_pid != pid) {
int p[2];
// new pipe
if (pipe(p) != 0) {
ABSL_RAW_LOG(FATAL, "Failed to create pipe, errno=%d", errno);
}
fcntl(p[0], F_SETFD, FD_CLOEXEC);
fcntl(p[1], F_SETFD, FD_CLOEXEC);
uint64_t new_pid_and_fds = Pack(current_pid, p[0], p[1]);
if (pid_and_fds.compare_exchange_strong(
local_pid_and_fds, new_pid_and_fds, std::memory_order_relaxed,
std::memory_order_relaxed)) {
local_pid_and_fds = new_pid_and_fds; // fds exposed to other threads
} else { // fds not exposed to other threads; we can close them.
close(p[0]);
close(p[1]);
local_pid_and_fds = pid_and_fds.load(std::memory_order_relaxed);
}
Unpack(local_pid_and_fds, &pid, &read_fd, &write_fd);
}
errno = 0;
// Use syscall(SYS_write, ...) instead of write() to prevent ASAN
// and other checkers from complaining about accesses to arbitrary
// memory.
do {
bytes_written = syscall(SYS_write, write_fd, addr, 1);
} while (bytes_written == -1 && errno == EINTR);
if (bytes_written == 1) { // remove the byte from the pipe
char c;
while (read(read_fd, &c, 1) == -1 && errno == EINTR) {
}
}
if (errno == EBADF) { // Descriptors invalid.
// If pid_and_fds contains the problematic file descriptors we just used,
// this call will forget them, and the loop will try again.
pid_and_fds.compare_exchange_strong(local_pid_and_fds, 0,
std::memory_order_relaxed,
std::memory_order_relaxed);
}
} while (errno == EBADF);
return bytes_written == 1;
}
} // namespace debugging_internal
ABSL_NAMESPACE_END
} // namespace absl
#endif